Lymphocytes, particularly T cells, undoubtedly have important interactions with hematopoietic cells both in the regulation of normal blood cell production and in the pathogenesis of some of the hematocytopenias. We and others have identified a subgroup of patients with neutropenia that have a striking excess of peripheral blood and bone marrow large granular lymphocytes (LGL) that are T3+, T8+, T11+, Fc receptor positive. Recently, 4 such patients from our institution have been shown to have clonal rearrangements of the T cell antigen receptor gene DNA. One particularly instructive case had spontaneous diminution of the T8+, LGL clone (determined by morphology, cell surface phenotype, and T cell receptor gene analysis), associated with complete remission and neutropenia and anemia. These observations support the hypothesis that the LGL are involved in the pathogenesis of the hematocyotpenia. In the first specific aim we propose to complete development of an in vitro method with which to demonstrate how LGL may be causally related to the neutropenia. Our previous studies have focused on possible LGL-granulocyte macrophage colony forming unit (CFU-GM) interactions and have not yielded meaningful results. Our current focus is on possible LGL involvement in mature neutrophil killing by means of ADCC. Other approaches are planned which include determining if LGL cell surface antigens, serum factors, or interferon are involved in the pathogenesis. Secondly, we will determine if other neutropenic patients with and without excess LGL subsets have clonal rearrangements of T cell antigen receptor gene DNA. In the third specific aim, we plan to perform a battery of in vitro tests including those established in specific aim 1, and assays established in other laboratories in evaluating all neutropenic patients. We will look for neutrophil antibodies, complement fixing antibodies to CFU-GM, T cell mediated contact and non-contact-dependent inhibition of CFU-GM or more neutrophils. The complete laboratory evaluation will be correlated with the clinical picture and response to proposed therapies in individual patients. These planned studies should advance our understanding of the pathogenesis of idiopathic neutropenia and be a first step toward rational rather than empiric therapy.